Waves can provide a reliable source of renewable energy compared to the solar and wind sources. There is a wide variety of wave energy extraction concepts depending on the mechanism of absorbing energy from the waves, the water depth, and the location (shoreline, near-shore, offshore). The energy extraction concepts can be categorized in three classes: oscillating water column devices, oscillating body systems, and overtopping converters. See A. F. de O. Falcao, Renew. Sustain. Energy Rev. 14(3), 899 (2010); M.-F. Hsieh et al., IEEE Trans. Sustain. Energy 3(3), 482, (2012); and A. Sproul and N. Weise, IEEE Trans. Sustain. Energy 6(4), 1183 (2015). The class of oscillating body systems includes single-body heaving buoys, two-body heaving systems, fully submerged heaving systems, and pitching devices. FIG. 1 is a widely used illustration of a typical heaving buoy (point absorber).
In point absorbers, the energy extraction results from the oscillating movement of a single body reacting against a fixed frame of reference (the sea bottom or a bottom-fixed structure). In one typical configuration of these Wave Energy Converters (WECs), hydraulic cylinders are attached to the floating body. When the float moves due to heave the hydraulic cylinders drive hydraulic motors which in turn drive a generator. See J. Falnes, Marine Struct. 20(4), 185 (2007). This type of WEC extracts the wave heave energy. There are other types of WECs that extract surge energy. See E. Renzi and F. Dias, Eur. J. Mech. B Fluids 41(0), 1 (2013). The mechanisms that translate the motion of oscillating bodies in water to useful electrical energy are usually called Power Take-Off (PTO) systems. See F. Fusco, Real-time Forecasting and Control for Oscillating Wave Energy Devices, Ph.D. thesis, NUI MAYNOOTH, Faculty of Science and Engineering, Electronic Engineering Department, July 2012.